The present invention relates to a circuit used in Time Division Multiple Access radio phone systems for Implementing a keyed switch-mode power supply.
In Time Division Multiple Access radio phone systems, the transmitter is on only part of the time, as for instance in the GSM system (Groupe Special Mobile) about one eighth of the time. Due to the internal resistance of the storage batteries variations in the operating voltage are created at the rate of the keying of the transmission. This causes disturbances in the functions of the transmitter and the receiver. Remedies to the situation can be made by coupling a large-size capacitor in parallel with the storage batteries.
The operating voltage variation caused by the charge state of the storage battery and the compatibility between various types of storage batteries also set limitations to the design and to the energy economy of a device. One known manner to reduce said problems is to use a switch type power source, i.e. a so-called Switched Mode Power Supply (SMPS) between the storage battery and the electronics.
A plurality of methods are known in generating a supply voltage with the aid of switching circuits. Such designs are useful in continuously power consuming products.
The prior art technology is described below referring to the accompanying FIG. 1 which presents a circuit known in the art for generating the operating voltage with the aid of a switching circuit. Various switch mode power supplies known in the art are described for example in reference "Advanced Linear European Seminars for Precision Design of the 90's", published by Texas Instruments UK, 1990, p. 59-82.
FIG. 1 presents a power source 1 supplying direct current to an SMPS 2. An SMPS includes an inductor to which the power is supplied via a switch. When the switched is opened a peak voltage is created by the inductor that charges a capacitor also within the SMPS. The capacitor smooths the output voltage.
Problems are caused by the peak voltages created in the SMPS when the switches therein are switched on and off with the current on. By an a-fold increase of the voltage of the storage battery, the current to be supplied by the storage battery increases a-fold relative to the current to the load. Also the power losses will increase.
In radio phones the above design has not, however, been used in general, primarily due to the electromagnetic interferences it causes, but also because of space constraints, and inefficiency. For high efficiency, the switching times of the power source need to be short, and this in turn increases interferences particularly in analog parts of the phone. In European patent application 92310967.2 "Voltage-Multiplexer" an alternative circuit is disclosed for producing an operating voltage by switching a capacitor charged by the battery into series with the battery to increase the supply voltage. However, in said design the variation in operating voltage caused by the charge state of the storage battery and compatibility problems related to various storage battery types cannot be totally eliminated.
In European patent application EP 239393 "Intermittently driven transmitter" an arrangement is disclosed to implement the power supply of a powerful transmitter with the aid of a capacitance. The design is not, however, as such, appropriate for use in less powerful time-divided radio phone systems.